You can enhance immune function through diet. The traditional medical systems of both China and Japan have used mushrooms for centuries as a means of increasing resistance to stress and promoting longevity. Historically, practitioners of traditional Chinese medicine weren't aware of the existence of the immune system as we know it today — a set of interacting organs, protective compounds, and cells — but they understood that the body has the means to defend itself and that certain mushrooms could help in this fight.
Today we know the active components of mushrooms are polysaccharides, very large, long-chain sugar molecules that are gobbled up by macrophages (large roving immune cells) in the gut wall and transported to other immune cells, setting off a chain of defensive events. The result is an increase in the number and activity of natural killer cells that protect us against bacteria and viruses and destroy malignant cells.
In addition to mushrooms, enjoy other immune-boosting foods such as several daily servings of fresh, organically grown fruits and vegetables. And because a growing body of evidence indicates that garlic and green tea have immune-enhancing effects, try to add both of them to your diet. Ideally, eat a clove of fresh garlic a day — raw, if you can, or lightly cooked. If you're a coffee drinker, substitute green tea for some or all of the coffee (if you prefer decaf, you can get decaffeinated green tea).
Be aware of the fat in your diet — avoid polyunsaturated vegetable oils, and foods that contain trans-fatty acids, such as partially hydrogenated oils, vegetable shortening, margarine, and deep-fried foods.
Drink several glasses of good-quality water each day to help keep your mucous membranes moist enough to fight off any viruses they encounter. And don't forget some other basic immune-strengthening tips such as washing your hands frequently, getting enough sleep, exercising regularly, and keeping a lid on stress.